Jet engines for powering aircraft obtain subsonic performance improvements by use of a fan, in essence a shrouded propeller. The fan is rotatably driven, through a shaft connection, by the turbine portion of the engine and serves to pass a large volume of air around the engine, thereby increasing overall engine thrust. In fact, in recent typical commercial fan engines, the fan moves several times as much air as is taken in by the engine compressor. More recently, extensive work has been directed toward varying the pitch of the fan blades. It is believed that many of the same advantages can be obtained as are available with variable pitch propellers since fans are, in reality, directly analogous to propellers with shrouds.
Accordingly, the variable pitch fan is being considered for application on high bypass turbofan engines for improved performance in several areas: noise reduction, thrust reversal, improved thrust response, and overall engine performance. For instance, full reversible variable pitch fans have been proposed for high bypass ratio engines on STOL-type (short take-off and landing) transports. Cascade-type and target-type thrust reversers which change the direction of the fan air flow passing around the engine (the bypass stream) become large and heavy so that it is highly desirable to find alternative means for generating decelerating thrust. It is possible to generate substantial reverse thrust in a high bypass ratio fan through fan blade pitch reversal which induces pumping of the air flow through the fan blading in the reverse direction.
It is possible to actuate the blades in either of two directions from their normal position to achieve reverse thrust. The blades may be actuated through feather (counterclockwise looking down at the blade tip of a rotor which is rotating clockwise aft looking forward) or through flat pitch (clockwise looking down at the blade tip). Actuating the blades through either position from the forward to the reverse thrust modes may cause the blade stresses to be unacceptably high. Actuation through feather induces high incidence angles (the difference between the air angle ahead of the fan in the relative frame and the fan blade leading edge metal angle, both measured from the axial direction) and as a result there is the danger of blade stall (i.e., the inability of the flow to remain attached to the fan blade with a resultant loss in fan pumping capability). Peak stresses occur just prior to clearing stall in the reverse thrust direction when actuating through feather. Actuation through flat pitch may cause lower peak blade stresses during the transition, but unacceptably high stresses may persist during the reverse thrust mode since the blade camber during reverse pitch is in the wrong direction. Since the camber is in the wrong direction, the fan blade is continually in at least a mild stall during this mode.
The problem facing the aerodynamic and mechanical designer, therefore, is that a fan blade designed for optimum performance in the forward thrust mode wil not be optimized for reverse thrust operation. Compromises which benefit reverse thrust operation invariably have been detrimental to forward thrust operation. The present invention will solve this problem by orienting the air flow into and out of the variable pitch fan blade in a predetermined relationship with the change in pitch of the fan in order to reduce aerodynamically induced stresses in the variable pitch fan blades as these blades change from the forward to the reverse thrust modes. While the present invention will aid the forward and reversed thrust steady-state operation, more importantly, it will also improve flow conditions and eliminate the stall problem during the fan blade transition. Prior state of the art attempts to solve this problem have been oriented toward strengthening the fan blade in order to withstand the high stresses. This results in thick, long-chorded blades, which are detrimental to fan aerodynamic performance, particularly in the hub region, and in added weight, which is obviously undesirable in any aircraft application.